Why does the fish constantly open its mouth and move its gill covers?

The mouth and gill covers take part in the respiration process of bony fish. When the fish opens its mouth and draws water into it, the gill covers are pressed. Then the fish closes its mouth and opens the gill covers, under the pressure of the tongue, water flows into the pharynx, which is pierced by the gill slits. Water comes out of them and washes the gills – gas exchange takes place: oxygen from the water goes into the blood, and carbon dioxide from the blood goes into the water. That is, the fish constantly opens its mouth and moves the gill covers, as it breathes.